1. Field of the Invention
The present invention concerns a Cu series lead frame material for use in resin-sealed semiconductor packages, having excellent heat dissipation in a state of mounting a semiconductor chip of large capacity and capable of preventing damages of the chip due to the disconformity of thermal expansion coefficient with that of the chip and, more in particular, it relates to a lead frame material for use in resin packages, in which metal foils of low thermal expansion are selectively clad in a spot-like pattern to a Cu series lead frame material at the surface on the side reverse to the surface where a semiconductor chip is to be mounted or at the surface where the semiconductor chip is to be mounted and at the surface on the side reverse to it, so that thermal bondability between the Cu series material of excellent heat dissipation and the semiconductor chip of large capacity can be improved and warpings of the substrate, etc. during heating upon mounting the chip is reduced, thereby preventing damages to the chip such as peeling or cracking.
2. Description of the Prior Art
Integrated circuit chips (hereinafter referred to as "chips") of semiconductor packages, especially, those in LSI or ULSI for large-scaled computers are getting to attain higher degree of integration and increased operation speed and, therefore, the quantity of heat to be generated by the operation of the devices has become extremely large because of the increase of the electric power to be consumed in the operation.
That is, since the capacity of the chips has been increased and the quantity of heat to be generated during the operation has also become large, if the thermal expansion coefficient of the substrate material is significantly different from that of the material of the chip made of silicon or gallium arsenic, there is a problem that the chip would suffer from peeling or cracking.
In view of the above, heat dissipation capacity is taken into consideration in the design of semiconductor packages and, therefore, a heat dissipation capacity is required also for a chip-carrying substrare and a large thermal conductivity is required for the substrate material.
In a resin-sealed semiconductor package as shown in FIG. 10, a lead frame serves not only as a path of electrically connecting a chip to the outside but also has an important role as a path for releasing the heat generated from the chip.
That is, in the plastic package, a chip 2 is mounted on an island 1.sub.1 formed in the central portion of a lead frame 1 and is fixed thereon with brazing material, adhesive or solder. The chip 2 is also electrically connected with an inner lead 1.sub.2 by way of a bonding wire 3 and, further, sealed therearound with a resin 4.
The heat to be generated from the chip 2 reaches an outer lead 1.sub.3 of the lead frame 1 by way of a path including the island 1.sub.1, the resin 4 and the inner lead 1.sub.2 and is then released to the outside along with the outer surface of the resin 4.
Accordingly, the lead frame 1 is desired to be made of a material having a high thermal conductivity so that heat generated from the chip can be released to the outside of the semiconductor package.
On the other hand, peeling of the chip 2 from the island 1.sub.1 occurring in the bonding interface therebetween and cracking formed in the resin 4 are caused by the difference of the thermal expansion coefficient of the chip 2 from that of the sealing resin 4 and the lead frame 1. In order to prevent such damages, the conformity of the thermal expansion coefficient of the chip 2 with that of the resin 4 and the lead frame 1 is indispensable.
As described above, for the lead frames in the semiconductor packages made of synthetic resin material, those made of copper alloys with good thermal conductivity have generally been used in view of the heat dissipation capacity.
However, for the application use requiring high reliability, copper alloys are not so favorable since they have a poor conformity with chips with respect to the thermal expansion coefficient and there may be a worry of peeling at the bonding interface between the chip and the island or cracking in the chip. Accordingly, it has been proposed a semiconductor package using Ni--Fe series alloy of low thermal expansion coefficient such as 42% Ni--Fe series alloy in view of the conformity of the thermal expansion coefficient relative to the chip.
However, since the Ni--Fe series alloy is poor in the thermal conductivity, no sufficient heat releasability as satisfying the present demand has yet been obtained. In addition, the difference of the thermal expansion between the chip and the sealing resin is extremely large. Accordingly, even if the conformity of the thermal expansion coefficient is satisfactory between the lead frame and the chip, the conformity between the lead frame and the resin is poor, so that it has been difficult to completely prevent the cracking in the sealing resin.